This project will evaluate novel therapeutic approaches to the treatment of acute leukemia and osteogenic sarcoma. This study will exploit the unique mechanisms of action of two novel antifolates for treatment of these malignancies. Studies in our laboratory have demonstrated that 50-60 percent of patients with acute lymphoblastic leukemia in relapse have defective transport as a mechanism of resistance and work in progress indicates that some osteosarcoma tumors have low levels of the reduced folate carrier. Therefore, the first approach will utilize trimetrexate, an antifolate which does not require utilization of the reduced folate carrier for cellular uptake, in combination with leucovorin protection in the treatment of patients with relapsed/refractory acute lymphoblastic leukemia and osteogenic sarcoma who demonstrate impaired methotrexate transport. Based upon studies in our laboratory demonstrating enhanced efficiency of polyglutamate formation in blasts of myeloid origin with raltitrexed ( Tomudex ) compared to methotrexate, the second approach will utilize this agent in the treatment of acute myelogenous leukemia. This approach will investigate a new target of inhibition, thymidylate synthase. The overall objective of the proposed studies will be the development of new treatment strategies for treatment of relapsed/refractory malignancies in adults and children. Both clinical studies will be linked to correlative laboratory studies directed toward evaluation of patient selection, optimization of treatment, mechanisms of resistance, and new drug development.